1. Field of the Invention
This invention is concerned with improvements in and relating to printing inks, especially printing inks for use in lithographic printing processes such as offset lithography.
2. Description of the Related Art
It is an essential requirement of inks for use in lithographic printing processes that they be oleophilic in character and, accordingly, in order to wash them from printing apparatus it has been considered necessary to use organic solvent systems as wash media. It has been proposed, e.g. in WO94/06873, to provide lithographic inks which can be washed from printing apparatus by the use of aqueous wash media, such inks containing at least one component containing free carboxylic acid groups so that the component can be rendered hydrophilic by complete or partial neutralization of the carboxylic acid groups, i.e. by using an alkaline aqueous wash medium. In the unneutralized form the component is sufficiently hydrophobic to make it suitable for use in a lithographic printing ink.
Typically, the carboxylic acid group-containing component used in the various lithographic inks particularly described in WO94/06873 is an alkyd resin having an acid value of 25 mgKOH/g or more and, in order to obtain washability, using an aqueous alkaline wash, for the ink as a whole, relatively large amounts of such alkyd resins have to be employed, with consequent restraints on flexibility of formulation often with a reduction in the potential printing properties of the resultant ink.